Because of their outstanding physical properties thermoplastic polyurethanes have been used for many years for a variety of different applications. Of the many processes described in the literature for producing thermoplasts of this type, direct production in self-cleaning twin-screw extruders is of particular interest. In machines of this type, the reaction mixture is homogenized very rapidly. In addition, the screws clean one another and the barrel in every zone, thereby avoiding even temporary caking of the reaction mixture which due to the prolonged effect of heat could lead to decomposition or to crosslinked products.
In conventional processes, special measures must be taken in order to combine the reactants (relatively high molecular weight polyol, chain extender and diisocyanate) in such a way that a homogeneous mixture is formed before the polyurethane formation begins to any significant extent. If the reactants are mixed too slowly, the polyurethanes formed are inhomogeneous and contain completely or substantially insoluble or infusible gel particles.
According to the teaching of U.S. Pat. No. 3,233,025, the individual reactants are combined initially in a special mixing unit and it is only the homogeneous reaction melt which is introduced into the screw extruder.
In the process described in U.S. Pat. No. 3,642,964, the reactants must be mixed very quickly. This result is obtained by intensively homogenizing the reaction melt by means of kneading elements provided in a mixing zone of a twin-screw extruder at the latest six seconds after the diisocyanate and diol have been introduced. In addition, in order to obtain a uniform product, the melt viscosity must remain substantially constant (between about 100 and 1000 Pa s) over the entire length of the extruder.
By contrast, according to the teaching of German Offenlegungsschriften, Nos. 2,302,564 and 2,549,372, the reaction mixture must be intensively mixed by means of kneading elements in the screw at a stage at which the reaction melt still has a low viscosity (approximately 20 to 70 Pa s) in order to prevent inhomogeneities from being formed in the end product.
In the processes according to German Offenlegungsschriften Nos. 2,447,368 and 2,610,980, a temperature profile decreasing from the feed zone to the output end of the extruder must be adjusted in the reaction melt. The production of a homogeneous thermoplastic polyurethane in accordance with the teaching of German Offenlegungsschrift No. 2,610,980 would appear to be particularly difficult because the starting products are first heated to a high temperature and only then are combined with one another. In the absence of rapid mixing, the polyurethane reaction would appear to take place to a considerable extent while local over-concentrations of the individual reactants are still present, so that a non-uniform product is formed. In addition, the starting temperatures are so high that the polyurethane formed can be thermally damaged.